Low sulphur diesel fuel and aviation turbine fuel

ABSTRACT

A process for the production of a synthetic low sulphur diesel fuel and a low soot emission aviation fuel is disclosed. The process includes fractionation of a Low Temperature Fischer-Tropsch feedstock into a light kerosene fraction and a heavier diesel fraction in a volumetric ratio of at least 1:2 to form the light kerosene fraction having a smoke point greater than 50 mm, a freezing point of below −47° C., a BOCLE lubricity wear scar less than 0.85 mm, and an anti-oxidant additiveless thermal stability tube deposit rating at 260° C. of less than 1 useable as a low soot emission aviation fuel and/or an aviation fuel blend stock, and the heavier diesel fraction having CFPP according to IP309 of below −5° C., a density@20° C. of at least 0.78 kg/l, and a viscosity@40° C. of above 2 cSt useable as a synthetic low sulphur diesel fuel and/or a diesel fuel blend stock.

This application is a continuation of U.S. patent application Ser. No.11/256,285 filed Oct. 19, 2005, now U.S. Pat. No. 7,390,397 which is acontinuation of International Patent Application PCT/ZA2004/000041 filedApr. 7, 2004.

FIELD OF THE INVENTION

The invention relates to a low sulphur diesel fuel and to an aviationfuel and a blending stock for aviation fuel.

BACKGROUND TO THE INVENTION

In this specification reference is made to Low TemperatureFischer-Tropsch (LTFT) process. This LTFT process is a well knownprocess in which carbon monoxide and hydrogen are reacted over an iron,cobalt, nickel or ruthenium containing catalyst to produce a mixture ofstraight and branched chain hydrocarbons ranging from methane to waxesand smaller amounts of oxygenates. This hydrocarbon synthesis process isbased on the Fischer-Tropsch reaction:

2H₂+CO→˜[CH₂]˜+H₂O where ˜[CH₂]˜ is the basic building block of thehydrocarbon product molecules.

The LTFT process is used industrially to convert synthesis gas, whichmay be derived from coal, natural gas, biomass or heavy oil streams,into hydrocarbons ranging from methane to species with molecular massesabove 1400. While the term Gas-to-Liquid (GTL) process refers to schemesbased on natural gas, i.e. methane, to obtain the synthesis gas, thequality of the synthetic products is essentially the same once thesynthesis conditions and the product work-up are defined.

While the main products are linear paraffinic materials, other speciessuch as branched paraffins, olefins and oxygenated components may formpart of the product slate. The exact product slate depends on reactorconfiguration, operating conditions and the catalyst that is employed,as is evident from articles such as Catal. Rev. -Sci. Eng., 23 (1&2),265-278 (1981) or Hydroc. Proc. 8, 121-124 (1982).

Preferred reactors for the production of heavier hydrocarbons are slurrybed or tubular fixed bed reactors, while operating conditions arepreferably in the range of 160-280° C., in some cases in the 210-260° C.range, and 18-50 bar, in some cases preferably between 20-30 bar.

The catalyst may comprise active metals such as iron, cobalt, nickel orruthenium. While each catalyst will give its own unique product slate,in all cases the product slate contains some waxy, highly paraffinicmaterial which needs to be further upgraded into usable products. TheLTFT products can be hydroconverted into a range of final products, suchas middle distillates, naphtha, solvents, lube oil bases, etc. Suchhydroconversion, which usually consists of a range of processes such ashydrocracking, hydrotreatment and distillation, can be termed a LTFTProducts Work-up process. Typically the process is normally configuredin such a way that only two liquid products are transferred to storage.In most instances a small amount of light hydrocarbons containing up tofour carbon atoms is also co-produced. The typical quality of the LTFTliquid products is presented in Table 1.

TABLE 1 Typical Quality of the LTFT Products LTFT Naphtha LTFT DieselDensity, kg/l (20° C.) 0.685 0.765 Distillation IBP, ° C. 54 151 T10, °C. 81 182 T50, ° C. 101 249 T90, ° C. 120 317 FBP, ° C. 131 334Composition, % wt n-paraffins 59.0 31.9 iso-paraffins 38.2 67.1Naphthenics ND ND Aromatics 0.3 ND Olefins 2.5 ND Oxygenates ND NDIso:Normal Paraffin ratio 0.65 2.10

The applicant has identified a need to utilise LTFT fuel, including GTLfuel, directly, without blending with cracked stocks, as a fuel thatwill be interchangeable with conventional diesel fuels.

Semi-synthetic aviation fuel was approved in 1999 under British AviationTurbine Fuel Defence Standard 91-91 (DEF STAN 91-91) specifications.

A need has thus been identified for a synthetic based fuel which meetsor exceeds the above standards and which permits use of LTFT products,including GTL products, or components thereof in the aviation industryas fuels and/or as blend stocks for fuels.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a processfor the production of a synthetic low sulphur diesel fuel and anaviation fuel from a Low Temperature Fischer-Tropsch (LTFT) feedstock,said process including the fractionation of the Low TemperatureFischer-Tropsch feedstock into a light kerosene fraction useable as anaviation fuel and/or an aviation fuel blend stock, and a heavier dieselfraction useable as a synthetic low sulphur diesel fuel and/or a dieselfuel blend stock, said fractions substantially complying with diesel andaviation fuel specifications.

Surprisingly, the diesel fuel meets the lubricity specification withoutthe use of additives, although being highly hydrogenated. Usually thoseskilled in the art would expect highly hydrogenated fuel to needlubricity improvers.

This has been identified as one method to increase the energy density ofLTFT Fuel and also adhering to the cold flow properties (CFPP—coldfilter plug point test) and lubricity specifications while producing alighter kerosene fraction that is useable either to blend with crude oilderived blending stock to produce a semi-synthetic aviation fuel ordirectly as a synthetic aviation fuel.

The process includes the fractionation and removal of at least 33 volume% of the LTFT feedstock to form said aviation fuel or blending stockhaving a final boiling point of about 270° C.

Typically, the process includes fractionation and removal of 45 volume%, or even 55 volume % of the feedstock.

The light kerosene fraction may be cut to meet the −47° C. freezingpoint of Jet A-1 at a cut point of 270° C. Again the lubricityproperties measured with the ball on cylinder lubricity evaluator(BOCLE) of the kerosene fraction was above expectations.

According to a second aspect of the invention, there is provided asynthetic low sulphur fuel or blend stock for a low sulphur fuel, saidfuel or blend stock having the following properties:

-   -   from 13 mass % to 17 mass % hydrogen;    -   iso:n-paraffins mass ratio of from 2 to 5    -   less than 0.1% m/m aromatics;    -   CFPP according to IP309 of below −5° C.;    -   density@20° C. of at least 0.780 kg/l; and    -   total oxygen content less than 80 ppm.

Typically the iso:n paraffins mass ratio is from 3 to 4.

The iso:n paraffins mass ratio may be 3.7.

The hydrogen may be about 15 mass % of the fuel or blend stock.

Typically the CFPP is below −9° C.

Surprisingly, the fuel meets the lubricity specification without the useof additives, although being highly hydrogenated.

Advantageously, the emission performance was not adversely affected whencompared to a crude derived low sulphur fuel even though its lighterends is removed.

The fuel or blend stock may be a LTFT diesel fraction.

The blend stock may have viscosity@40° C. of above 2 cSt.

The fuel or blend stock may have a final boiling point of above 330° C.,typically about 340° C.

The fuel or blend stock may have an IBP of above 200° C., typicallyabove 250° C., in some embodiments in excess of 265° C.

According to a third aspect of the invention, there is provided asynthetic aviation fuel or fuel blend stock for a semi-syntheticaviation fuel, said blend stock having the following properties:

-   -   from 13 mass % to 17 mass % hydrogen;    -   iso:n-paraffins mass ratio of 0.5 to 3;    -   BOCLE lubricity wear scar less than 0.85 mm;    -   oxygen as oxygenates less than 50 ppm; of which    -   oxygen as primary C7-C12 alcohols is less than 50 ppm; and    -   oxygen as primary C12-C24 alcohols is less than 50 ppm.

The oxygen as oxygenates may be less than about 10 ppm.

The oxygen as primary C7-C12 alcohols may be less than about 10 ppm.

The oxygen as primary C12-C24 alcohols may be less than about 10 ppm.

The synthetic aviation fuel or fuel blend stock may have less than 0.1%m/m aromatics according to HPLC.

The synthetic aviation fuel or fuel blend stock may have a smoke pointgreater than 50 mm.

The synthetic aviation fuel or fuel blend stock may have a density@20°C. about 0.75 kg/l.

The synthetic aviation fuel or fuel blend stock may have a freezingpoint of below −47° C.;

Typically the iso:n paraffins mass ratio is from 1 to 2.

The iso:n paraffins mass ratio may be 1.2, or 1.16.

The hydrogen may be about 15 mass %.

The blend stock may be used directly as a fully synthetic aviation fuelwithout blending with crude derived fuel components.

The blending stock may be a LTFT kerosene fraction.

The blend stock may have viscosity@−20° C. less than 8 cSt, typically 4cSt.

The blend stock may have a final boiling point of above 200° C.,typically about 270° C.

According to a fourth aspect of the invention, there is provided asemi-synthetic aviation fuel including a blending stock as describedabove having the following properties:

-   -   iso:n-paraffins ratio of 0.5 to 3;    -   Smoke point greater than 35 mm; and    -   at least 8% m/m aromatics.

The semi synthetic aviation fuel may have a density@15° C. of at least0.775 kg/l.

The semi synthetic aviation fuel may have a smoke point greater than 50mm.

The semi synthetic aviation fuel may have a freezing point of below −47°C.;

Typically the iso:n paraffins mass ratio is from 1 to 2.

The iso:n paraffins mass ratio may be 1.8.

The blend stock may have viscosity@−20° C. of below 8 cSt, or even below4 cSt.

With a 50 vol-% blend of LTFT kerosene and crude derived sweetened andseverely hydrotreated kerosene, the minimum density and aromatic contentrequirements according to the American Society for Testing and Material(ASTM D1655) and the British Aviation Turbine Fuel Defence Standard91-91 for Jet A-1 were met.

Since LTFT fuel is composed almost only of normal and isoparaffins, aLTFT kerosene fraction may be utilised as an aviation turbine fuelblending component. The virtual absence of aromatics and naphthenes fromLTFT kerosene may provide it with a very good smoke point number (i.e.it produces very little soot).

According to a fifth aspect of the invention there is provided athermally stable aviation fuel with low deposition tendency whencombusted, said fuel including one or more fuel selected from a fullysynthetic aviation fuel, a semi synthetic aviation fuel, and a syntheticaviation fuel blend stock, as described above.

Typically the aviation fuel and blend stock has a thermal stability tubedeposit rating at 260° C. less than 1.

Typically the aviation fuel has a Quartz Crystal Microbalance (QCM)deposition less than 3 μg/cm².

More typically, the aviation fuel has QCM deposition of less than 2μg/cm² for a 15 h QCM test @ 140° C. without addition of ananti-oxidant.

According to a sixth aspect of the invention there is provided a lowsoot emission aviation fuel, said fuel including one or more fuelselected from a fully synthetic aviation fuel, a semi-synthetic aviationfuel, and a synthetic aviation fuel blend stock, as described above.

Typically the aviation fuel blend stock has about a 33% reduction in thenormalized particulate number density under cruise conditions, moretypically a 60% reduction under cruise conditions and a 67% reduction inthe normalized particulate number density under idle conditions, moretypically a 83% reduction under idle conditions compared to typicalconventional aviation fuel.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention will now be described with reference to specificembodiments which illustrate the invention but are not intended to limitits application.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows Gas Chromatograph Mass Spectrometry (GC MS) and GasChromatograph Flame Ionisation Detection (GC-FID) results on Sasol SPD™diesel;

FIG. 2 shows that fractionation of Sasol SPD™ diesel results in a dieselhaving a higher density;

FIG. 3 shows the composite HC emissions of various fuels;

FIG. 4 shows CO emissions of various fuels;

FIG. 5 shows composite NOx emissions of various fuels;

FIG. 6 shows composite particulate matter (PM) emissions of variousfuels;

FIG. 7 shows composite CO₂ emissions of various fuels;

FIG. 8 shows fuel compositions of various fuels;

FIG. 9 shows 55 vol-% heavy cut Sasol SPD™ diesel emissions vs EN590;

FIG. 10 shows 55 vol-% heavy cut Sasol SPD™ diesel emissions vs fullrange Sasol SPD™ diesel; and

FIG. 11 shows distillation profiles of sweetened Kero Merox™ kerosene,the 45% Sasol SPD™ kerosene fraction and a 50% blend thereof.

EXAMPLES

Low Sulphur Diesel Fuel

Sasol Slurry Phase Distillate™ diesel or Sasol SPD™ diesel wasfractionated targeting a freezing point requirement for Jet A-1 of −47°C. with a final boiling point of 270° C. The resultant diesel fuelproperties and kerosene properties are shown in Table 2 and includedensity, viscosity, high-frequency reciprocating rig (HFRR) andball-on-cylinder evaluator (BOCLE) lubricity test and cold filterplugging point (CFPP) of diesel and freezing point of kerosene.

TABLE 2 Selected fuel properties of Sasol SPD ™ diesel, the heavierdiesel fraction and the lighter kerosene cut obtained throughfractionation. Sasol Sasol SPD ™ Sasol SPD ™ Sasol SPD ™ Sasol SPD ™SPD ™ Diesel Diesel 1 kerosene 1 Diesel 2 kerosene 2 Product Yield vol %100% 45% 55% 55% 45% Density @ 15° C. ASTM kg/l 0.769 0.790 0.752 0.7860.747 D4052 Density @ 20° C. ASTM kg/l 0.765 0.786 0.748 0.782 0.743D4052 H₂ content ASTM mass % 14.97 14.62 15.00 14.68 14.87 D5291Distillation IBP ° C. 151 268 157 246 154 ASTM D86 10% ° C. 182 282 172262 168 50% ° C. 249 300 203 289 191 95% ° C. 325 336 267 333 254 FBP °C. 334 341 278 339 267 Flash point ASTM ° C. 58 114 50 122 45 D93Freezing point ASTM ° C. −15 −42 −48 5901 CFPP IP 309 ° C. −19 −9 −12Viscosity @ −20° C. ASTM cSt 4.26 4.17 D445 Viscosity @ 40° C. ASTM cSt2.00 3.90 3.31 D445 Lubricity (HFRR) ASTM μm 617 450 751 446 781 D6079Lubricity ASTM mm 0.81 0.83 (BOCLE) D5001 Cetane number ASTM 72 76 75D613 Gross Heating ASTM MJ/kg 46.96 46.55 47.13 46.97 47.24 Value D240Net Heating ASTM MJ/kg 43.79 43.44 43.95 43.86 44.09 Value D240 NetHeating ASTM MJ/l 33.50 34.13 32.88 34.25 32.75 Value D240 Oxygen asGC-MS ppm <6 <6 <6 <6 <6 oxygenates Oxygen as primary GC-MS ppm <6 <6 <6<6 <6 C7-C12 alcohols Oxygen as primary GC-MS ppm <6 <6 <6 <6 <6 C12-C24alcohols Total oxygen GC-TCD ppm <80 <80 <80 <80 <80 In table 2: SasolSPD ™ diesel is the full distillation range LTFT diesel Sasol SPD ™diesel 1 is a 45% heavy end LTFT diesel Sasol SPD ™ diesel 2 is a 55%heavy end LTFT diesel Sasol SPD ™ kerosene 1 is a 55% light end LTFTkerosene cut Sasol SPD ™ kerosene 2 is a 45% light end LTFT kerosene cut

TABLE 3 Selected fuel properties of the Sasol SPD ™ kerosene fractionblends with crude derived sweetened and hydrotreated kerosene. 50/5050/50 Sasol Kero Kero DHC DEF STAN SPD ™ Merox ™ Merox DHC kerosene91-91 Property Units kerosene kerosene blend Kerosene blend min maxTotal Aromatics vol % 0 19.8 9.9 12.7 6.5 8 25 Olefins vol % 0 0 0 0 0Paraffins vol % 99.9 80.2 90.1 87.3 93.5 Total Sulphur Mass % <0.01 0.140.07 <0.01 <0.01 0.3 Mercaptan Sulphur Mass % 0.0002 0.0006 0.00040.0005 0.0003 0.003 Doctor test Negative Negative Negative NegativeNegative Negative Total acid mgKOH · g 0.009 0.002 0.009 <0.001 0.010.015 Density @ 15° C. kg/l 0.747 0.809 0.776 0.820 0.784 0.775 0.840 @20° C. kg/l 0.743 0.805 0.772 0.817 0.780 Distillation D86 IBP ° C. 154158 152 184 156 10% ° C. 168 176 169 195 179 205 50% ° C. 191 206 194218 204 95% ° C. 254 259 253 274 268 FBP ° C. 267 267 267 280 278 300Flash point ° C. 45 55 48 52 53 38 Viscosity @ −20° C. cSt 4.51 3.655.33 4.61 8 Freezing point ° C. −48 −48 −51 −51 −50 −47 Lubricity(BOCLE) mm 0.83 0.48 0.79 0.68 0.85 0.85 Thermal Stability Filterpressure drop mmHg 0 0 0 0 0 25 Tube deposit rating visual <1 <1 <1 <1<1 <3 Contaminants Microsep without SDA rating 75 95 74 84 80 85 Waterinterfase rating rating 1b 1b 1b 1b 1b 1b Combustion Smoke point mm >5025 36 27 37 25 Specific energy MJ/kg 44.09 42.8Diesel Fractions

The 45 vol-% heavy end diesel fraction has excellent properties for useas a neat Sasol SPD™ diesel without the use of additives

A maximum wear scar diameter (WSD) of 460 μm is allowed according to theEN 590:1999 Diesel Fuel Specifications. The lubricity of the Sasol SPD™diesel fractions with a total oxygen content less than 80 ppm increasedconsiderably and meets the current specification requirement because ofthe higher viscosity of the diesel fractions, which improve thehydrodynamic lubrication without the use of a lubricity improver.

The flash points of the Sasol SPD™ diesel fractions are high because ofits higher initial boiling point whereas the cold flow properties of thediesel fraction remained good.

According to Gas Chromatograph Mass Spectrometry (GC MS) and GasChromatograph Flame Ionisation Detection (GC-FID) results Sasol SPD™diesel, prior to fractionation to kerosene and a diesel, has anisoparaffin to normal paraffin ratio of 2.2:1 (see FIG. 1). The 55%heavy end diesel cut has a isoparaffin to normal paraffin ratio of 3.71.

Fractionation of Sasol SPD™ diesel results in a diesel having a higherdensity (see FIG. 2) and energy density which results in better fueleconomy or more power. It also revealed other changes afterfractionation including an improvement in its lubricity, a much higherviscosity and flash point. The good cold flow properties did notdecreased dramatically although the diesel fraction is much heavier.

Exhaust Emission Performance Of the Heavy Cut Diesel

The exhaust emissions of a heavy cut of Sasol SPD™ diesel fuel werecompared with those of the full boiling range Sasol SPD™ diesel, as wellas a European reference diesel fuel. The tests were performed using alate model European passenger car. It was found that the emissionperformance was not adversely affected when compared to the conventionaldiesel conforming to current EN590 fuel specifications, althoughunburned hydrocarbons, carbon monoxide, and particulate matter emissionsdeteriorated when compared with the full boiling range Sasol SPD™diesel. The higher volumetric energy content of the heavy cut Sasol SPD™diesel resulted in an improvement of 2% in the measured fuel consumptionwhen compared to the full boiling range Sasol SPD™ diesel.

Test vehicle Model: 2002 BMW 320d sedan Test Mass: 1 474 kg Enginedisplacement: 1 995 cm³ Bore/stroke: 84/90 mm Compression ratio: 17:1Power output: 110 kW @ 4000 rev/min Maximum torque: 330 Nm @ 2000rev/min Fuel injection system: Bosch common rail Exhaust aftertreatment:Dual oxidation catalysts Emission certification: EU 3 (2000)Test Fuels

Three fuels were tested for the comparison:

-   EN590: A conventional diesel fuel meeting the European EN 590    specification, and with a sulphur content of <10 mg/kg.-   Full boiling range Sasol SPD™ diesel: Sasol SPD™ diesel with an IBP    of 150° C. and FBP of 335° C.-   55 vol-% heavy Sasol SPD™ diesel cut: A heavy cut of Sasol SPD™    diesel, comprising of the remainder after a 45 vol-% kerosene    fraction had been removed by fractionation.

Relevant fuel specifications are provided in Table 2 above:

The emission tests performed according to European EC/ECE test method,and using the NEDC test cycle. Two preconditioning runs were performedin preparation for each test. Three tests were performed with each ofthe EN590 and full boiling range Sasol SPD™ diesel, and two tests wereperformed with the 55 vol-% heavy Sasol SPD™ diesel cut. The fuels weretested sequentially, and the vehicle was warmed up and run at a speed of120 km/h for a period of 5 minutes after each fuel change.

The results for the ECE R15 urban cycle, the EUDC highway cycle, and thecombined ECE R15+EUDC cycle, are presented in Tables 4, 5, and 6 below.

TABLE 4 ECE R15 Urban Cycle Emissions and Fuel Consumption ExhaustEmissions (g/km) FC Fuel CO HC NO_(x) PM CO₂ (l/100 km) EN590 Avg. 0.4190.077 0.391 Not 220.4 8.40 Std Dev 0.078 0.019 0.012 measured 3.6110.141 COV (%) 18.6 25.2 3.0 1.6 1.7 Full range Avg.. 0.113 0.031 0.388212.7 8.89 Sasol SPD ™ Std Dev. 0.011 0.005 0.011 1.246 0.051 diesel COV(%) 9.4 14.8 2.9 0.06 0.6 Heavy Avg. 0.152 0.035 0.371 211.7 8.64 SasolSPD ™ Std Dev 0.003 0.001 0.000 0.896 0.037 diesel COV (%) 1.7 3.8 0.10.4 0.4

TABLE 5 EUDC Highway Cycle Emissions and Fuel Consumption ExhaustEmissions (g/km) FC Fuel CO HC NO_(x) PM CO₂ (l/100 km) EN590 Avg. 0.0080.003 0.295 Not measured 127.2 4.84 Std Dev 0.001 0.000 0.001 1.3630.052 COV (%) 15.6 12.4 0.4 1.1 1.1 Full Avg.. 0.010 0.003 0.283 123.15.14 range Std Dev. 0.004 0.000 0.008 1.071 0.045 Sasol COV (%) 35.310.8 2.9 0.9 0.9 SPD ™ diesel Heavy Avg. 0.008 0.003 0.279 123.2 5.03SPD ™ Std Dev 0.000 0.000 0.009 0.097 0.004 diesel COV (%) 5.2 15.7 3.10.1 0.1

TABLE 6 Composite ECE + EUDC Cycle Emissions and Fuel ConsumptionExhaust Emissions (g/km) FC Fuel CO HC NO_(x) PM CO₂ (l/100 km) EN590Avg. 0.159 0.030 0.330 0.025 161.4 6.15 Std Dev 0.029 0.007 0.004 0.0002.064 0.080 COV 18.1 23.9 1.3 1.6 2.2 1.3 (%) Full Avg.. 0.048 0.0130.322 0.020 156.0 6.52 range Std 0.002 0.002 0.009 0.001 1.118 0.047Sasol Dev. SPD ™ COV 3.3 14.8 2.9 4.4 3.0 0.7 diesel (%) Heavy Avg.0.061 0.015 0.313 0.027 155.8 6.36 SPD ™ Std Dev 0.000 0.000 0.005 0.0000.378 0.015 diesel COV 0.7 1.3 1.7 1.0 1.7 0.2 (%)

The results are also presented graphically in FIGS. 3 to 8.

The following may be concluded from the emission tests performed:

-   -   Use of the heavy cut of Sasol SPD™ diesel fuel did not adversely        affect the exhaust emissions of the test vehicle, when compared        to a European EN590 reference diesel fuel. HC and CO emissions        were lower than the EN590 fuel, while NOx and particulate        emissions where similar. All regulated emissions were well        within the Euro 3 limits for which the test vehicle is        certified.    -   Removing the lighter 45% of the diesel results in increases in        HC, CO and PM emissions, when compared to the full boiling range        diesel. While HC and CO emissions are still lower than with the        EN590 reference fuel, PM emissions were similar to the EN590        fuel, and some 30% higher than the full boiling range diesel.    -   The increased density of the heavy cut of the Sasol SPD™ diesel        results in an improvement in volumetric fuel consumption of 2%,        when compared to the full boiling range diesel. Fuel consumption        is still some 3% higher than with the EN590 diesel fuel,        however.        Aviation Fuel

The above tables and discussion regarding the low sulphur diesel fuelrefer. Viscosity and freezing point are the physical properties used toquantitatively characterise aviation fuel fluidity and only an upperviscosity limit is therefore specified for aviation fuel to which thefully synthetic Sasol SPD™ kerosene fractions conform. The light 45vol-% Sasol SPD™ fully synthetic kerosene fraction met the requiredfreezing point of −47° C. for Jet A-1 according to the DEF STAN 91-91with a freezing point of −48° C. (see Table 2). The low freezing point,determined in accordance with the automated ASTM 5901 test method, isbelieved to be attributable to the more than 60 mass-% iso-paraffinspresent in the full range Sasol SPD™ diesel and more than 50 mass-%iso-paraffin present in the fully synthetic Sasol SPD™ kerosene cut.

The amount of energy contained in a given quantity of fuel is importantsince space comes at a premium in an aircraft. A fuel with a highvolumetric energy content maximised the energy that can be stored in afixed volume and thus provides the longest flight range. The specifiednet gravimetrical energy content of the Sasol SPD™ kerosene fractionsare greater than the specified 42.8 MJ/kg (see Table 3).

The lubricity of the fully synthetic Sasol SPD™ kerosene cut, evaluatedwith the Ball-on-Cylinder Evaluator (BOCLE) (ASTM D5001 test method),has an unexpected wear scar diameter less than the maximum-wear scardiameter that is specified for Jet A-1.

Sweetened Crude Derived Kerosene Blend with Sasol SPD™ Kerosene

According to the specific approval of semi-synthetic jet fuel as JetA-1, its aromatic content must not be less than 8 vol-%. With Sasol SPD™diesel containing no aromatics (<0.001 mass-%), the 45 vol-% Sasol SPD™kerosene cut was blended in a 50/50 ratio with sweetened crude derivedkerosene from Merox™. The properties of the fully synthetic Sasol SPD™kerosene as blending stock (see Table 3) and an example of sweetenedkerosene, Kero Merox™, and a blend thereof are also summarised in Table3.

The sweetened Merox treated crude derived kerosene used for the blendhad a density of 0.809 kg/l @ 15° C. and the semi-synthetic blend had aboundary specified density of 0.776 kg/l @ 15° C. The aromatic contentof the blend was beyond the 8 vol-% limit (see Table 3).

The composition, volatility, fluidity, water separation characteristics,lubricity and thermal stability (JFTOT) requirements for semi-syntheticjet fuel are met with up to a 50 vol-% sweetened crude derived kerosenestream—Sasol SPD™ kerosene blend. The distillation profile of the blendis shown in FIG. 11.

Synthetic kerosene blends with crude derived jet fuel have already beenapproved with certain limitations. These include synthetic kerosenederived solely from the Fischer-Tropsch process without the inclusion ofsynthetic aromatic compounds. The light Sasol SPD™ kerosene with a finalboiling point of 270° C. conforms to these limitations and also to thefreezing point requirement for Jet A-1 of −47° C. As a blend, itsdensity and aromatic content will also conform to the minimumrequirement of 0.775 kg/l @ 15° C. and an 8 vol-% aromatic content.

Severely Hydrotreated Crude Derived Kerosene Blend with Sasol SPD™Kerosene

Up to 50 vol-% blends of Sasol SPD™ diesel with severely hydrotreatedcrude derived kerosene were also prepared to demonstrate a thermallystable semi-synthetic jet fuel conforming to Jet A-1 requirements suchas freezing point, density and lubricity. The properties of a 50 vol-%blend with a severely hydrotreated kerosene, a Distillate Hydrocrackedkerosene as example, is shown in Table 3.

Thermal Stability

The thermal oxidation stability of the fully synthetic aviation fuel andsemi synthetic aviation fuel (blends of the light Sasol SPD™ kerosenefractions with sweetened and severely hydrotreated crude derivedkerosene) were determined according to the jet fuel thermal oxidationtester (JFTOT) ASTM D3241 test method. The visual tube deposite ratingfor the fully as well as the semi-synthetic aviation fuel were less than1 with no pressure drop across the filter.

Thermal stability results with the Quartz Crystal Microbalance (QCM)confirmed the JFTOT results with only 2 μg/cm² deposition observed afterthe 15 hour test at 140° C. without the presence of anti-oxidants.

Tests with the JP-8+100 thermal stability improving additive did notimprove the stability of the synthetic aviation fuel and blends thereofsince the fuel is such a low depositor.

Soot Emissions

Gas turbine engine tests results on particulates (soot) of the fullysynthetic aviation fuel and blends thereof under idle and cruiseconditions were compared with that of typical convention aviation fuel.The fully synthetic Sasol SPD™ light kerosene cut formed 40% less sootunder cruise conditions than conventional JP-8 aviation fuel whereas ablend thereof formed 33% less soot under cruise conditions.

Under idle conditions, the Sasol SPD™ kerosene blend stock formed 83%less soot compared to typical convention aviation fuel, whereas blendsthereof formed 67% less soot.

1. A synthetic aviation fuel or fuel blend stock for a semi-syntheticaviation fuel, said aviation fuel or blend stock having the followingproperties: from 13 mass % to 17 mass % hydrogen; iso:n-paraffins massratio of 0.5 to 3; BOCLE lubricity wear scar less than 0.85 mm; andoxygen as oxygenates less than 50 ppm; of which oxygen as primary C7-C12alcohols is less than 50 ppm; and oxygen as primary C12-C24 alcohols isless than 50 ppm.
 2. A synthetic aviation fuel or fuel blend stock asclaimed in claim 1, having less than 0.1% m/m aromatics.
 3. A syntheticaviation fuel or fuel blend stock as claimed in claim 1, having a smokepoint greater than 50 mm.
 4. A synthetic aviation fuel or fuel blendstock as claimed in claim 1, having a density@20° C. about 0.75 kg/l. 5.A synthetic aviation fuel or fuel blend stock as claimed in claim 1,having a freezing point of below −47° C.
 6. A synthetic aviation fuel ora fuel blend stock as claimed in claim 1, wherein the iso:nparaffinsmass ratio is from 1 to
 2. 7. A synthetic aviation fuel or a fuel blendstock as claimed in claim 1, wherein the iso:nparaffins mass ratio isfrom 1.16 to 1.2.
 8. A synthetic aviation fuel or a fuel blend stock asclaimed in claim 1, wherein the hydrogen is about 15 mass %.
 9. Asynthetic aviation fuel or a fuel blend stock as claimed in claim 1,which is a Low Temperature Fisher-Tropsch Process (LTFT) kerosenefraction.
 10. A synthetic aviation fuel or a fuel blend stock as claimedin claim 1, wherein the blend stock has a viscosity@−20° C. of less than8 cSt.
 11. A synthetic aviation fuel or a fuel blend stock as claimed inclaim 1, which has a final boiling point of above 200° C.
 12. Asemi-synthetic aviation fuel including from 0.1 mass % to 99.9 mass % ofa blending stock as claimed in claim 1, said semi-synthetic aviationfuel having: iso:n-paraffins ratio of 0.5 to 3; Smoke point greater than35 mm; and at least 8% m/m aromatics.
 13. A semi-synthetic aviation fuelas claimed in claim 12, having a density@15° C. of at least 0.775 kg/l.14. A semi-synthetic aviation fuel as claimed in claim 12, having afreezing point of below −47° C.
 15. A semi-synthetic aviation fuel asclaimed in claim 12, having an iso:nparaffins mass ratio of from 1 to 2.16. A semi-synthetic aviation fuel as claimed in claim 15, wherein theiso:nparaffins mass ratio is 1.8.
 17. A semi-synthetic aviation fuel asclaimed in claim 12, including 50 vol % of the blending stock, whereinthe blending stock is LTFT kerosene, and 50 volume % crude oil derivedsweetened and severely hydrotreated kerosene.
 18. A thermally stableaviation fuel with low deposition tendency when combusted, said fuelincluding one or more fuel selected from a synthetic aviation fuel, anda synthetic aviation fuel blend stock as claimed in claim 1, and anothersemi-synthetic aviation fuel, said thermally stable aviation fuel havinga thermal stability tube deposit rating at 260° C. of less than
 1. 19. Athermally stable aviation fuel as claimed in claim 18, having QuartzCrystal Microbalance (QCM) deposition of less than 3 μg/cm².
 20. Athermally stable aviation fuel as claimed in claim 18, having QCMdeposition of less than 2 μg/cm² for a 15 h QCM test @140° C. withoutaddition of an anti-oxidant.
 21. A synthetic aviation fuel or fuel blendstock for a semi-synthetic aviation fuel, as claimed in claim 1, whereinthe oxygen as oxygenates is less than about 10 ppm.
 22. A syntheticaviation fuel or fuel blend stock as claimed in claim 1, wherein theoxygen as primary C7-C12 alcohols is less than about 10 ppm.
 23. Asynthetic aviation fuel or fuel blend stock as claimed in claim 1,wherein the oxygen as primary C12-C14 alcohols is less than about 10ppm.
 24. A synthetic aviation fuel or a fuel blend stock as claimed inclaim 11, wherein the final boiling point is about 270° C.